Aircraft typically include a plurality of flight control surfaces (fixed wing aircraft) or one or more cyclic pitch control swashplates (rotary wing aircraft) that, when controllably positioned, guide the movement of the aircraft from one destination to another. The number and type of flight control surfaces or swashplates included in an aircraft may vary.
The positions of the aircraft flight control surfaces or swashplates are typically controlled using a flight control actuation system. The flight control actuation system, in response to position commands that originate from either the flight crew or an aircraft autopilot, moves the aircraft flight control surfaces or swashplate(s) to the commanded positions. In most instances, this movement is effected via actuators that are coupled to the flight control surfaces or swashplate(s). Though unlikely, it is possible that a flight control surface actuator could become jammed, uncontrollably free, or otherwise inoperable. Thus, some flight control surface actuation systems are implemented with redundant actuators.
Redundant flight control actuators that have two hydraulic systems typically implement one of two operational configurations—an active-standby configuration or an active-active configuration. With the active-standby configuration, one actuator is actively powered while the other is in a standby mode. With the active-active operational configuration, both of the actuators are simultaneously powered. This latter operational configuration provides certain advantages over the active-standby configuration. Specifically, the active-active approach allows each individual actuator to be sized relatively smaller as compared to the actuators used to implement the active-standby configuration. Additionally, redundancy management becomes simpler. It is noted, however, that the active-active operational mode does present the potential for a resultant force-fight between the active actuators.
Electro-Hydraulic Servo-Valves (EHSVs) are very high pressure gain devices. When two hydraulic actuators are rigidly connected together, as in a redundant actuator, and operated simultaneously by independent electronic channels, a very small mismatch in command can create full force fight between the two actuators. The force-fight results from the fact that the actuators, position sensors, control electronics, and mechanical components have independent, unique tolerances. In the case of side by side hydraulic stages, the resultant effect of force fighting is severe bending in both piston rods, creating a racking effect in the actuator, which can accelerate structural fatigue and bearing wear. Designing a redundant actuator to withstand the worst-case wear and fatigue that could occur in the active-active operational configuration would result in additional weight, and associated costs.